The fires in and around Los Angeles have already claimed dozens of lives, destroyed thousands of homes and led to evacuation orders for hundreds of thousands of people. The economic damage is expected to be up to $150 billion. Daniel Swain is a climate scientist at UCLA and the University of California Agriculture and Natural Resources who studies extreme weather events and their connection to climate change. He and I recently spoke on the phone. Below is our conversation, edited for length and clarity. In it, we discuss what really caused these fires to get out of control, how he tries to convey the role of climate change in natural disasters, and how the Los Angeles area became so vulnerable to disasters.

What makes these fires unique other than the extent of the damage they caused?

I think the sheer scale of the damage in terms of the number of buildings destroyed and the economic losses is what stands out most. However, there are other aspects of the overall situation that are very different from anything we have seen before.

Let’s start with what’s not unusual, as I think this is helpful in preparation. It is not uncommon for strong dry winds to occur in the mountains and valleys near Los Angeles in January. This particular event was particularly powerful and spanned more populated and lower-lying regions than usual. But if you were to pick one time of year to expect strong, dry winds, it would be now.

These are the Santa Ana winds you’re talking about?

Yes, this is similar to Santa Ana. These were a little different from a traditional Santa Ana event in that they were driven by a slightly different low pressure system in a slightly different location. That’s partly why the winds come stronger from the north than the east, but also because they were stronger than usual and reached deeper into the valleys than usual. But if you think of it as Santa Ana or an Santa Ana is adjacent, it’s the same basic idea. We only see winds this strong about every five to ten years. So it’s certainly notable, but in itself it’s far from unprecedented.

However, the conditions were much more unusual and bordered on unprecedented – especially the drought. And that’s essentially quantified by looking at how much rain did or didn’t fall in the Los Angeles or San Diego area. And we note that this is now either the driest or second driest start to the season on record across Southern California in a hundred years. In modern history it has never been so dry this late in the supposed rainy season.

That’s something that sets the stage for these fires because if the same winds so far in the season had occurred after, say, an inch or two of rain, it would still be a good soaking, even if that’s below average. If that had happened, we wouldn’t be seeing the fires we’re currently seeing. We wouldn’t have this explosively dry vegetation. There has been essentially no rain in Los Angeles since last spring – only about a tenth of an inch or less in many areas, which is insignificant from a wildfire perspective.

In addition, the inland parts of Southern California – the mountains, the high plains and the desert regions – experienced the hottest summer on record. To be clear, the city of Los Angeles did not do this. But then, in early September, even the city of Los Angeles and actually the entire Los Angeles Basin experienced a record-breaking heat wave. And if you remember, that was actually connected to the big forest fire that broke out at the beginning of September. Many structures were destroyed. It remained unseasonably warm and it hasn’t rained at all since then. That’s the real anomaly here, as the winds are sort of a second order anomaly.

Anyone can understand why a dry branch is easier to burn. But is that too simplistic a way to think about why drought leads to fires that can burn out of control? Is there another level at which it is dangerous?

There are different levels, although I think basic intuition gets a good part of the way there. Imagine trying to light a damp log for a campfire. That won’t happen. There’s this binary switch where if the vegetation is too wet, combustion won’t occur. There will be no flame. When you light a match, it sprays out. You can’t light your campfire.

Then there is another level of dryness where the wood is not moist, but not particularly dry either. It’s somewhere in the middle. And if you’re really good at starting a campfire, you can probably get the wood going with a little preheating, right? So if you take a few twigs and some grass, the wood will eventually catch because the flame underneath will dry it out fairly adequately.

But there is also another level of drought where the vegetation is extremely susceptible to a spark. And not only does it increase the chances of it igniting in the first place, but it also greatly increases the actual intensity of the subsequent burn or fire. And by intensity I literally mean the amount of heat energy it gives off. This is very important because it not only determines how intense the flames are and how much damage they can cause locally, but also how quickly these flames can spread. In general, the more intense the fire, the faster it can spread. It also increases the fire’s tendency to create its own local weather conditions, which can be somewhat self-reinforcing.

So there are these self-reinforcing vicious circle feedbacks. Vegetation may be too wet to even burn. In this case, the risk of a forest fire is almost zero, right? And then there’s this other state where it’s dry enough to burn, but maybe doesn’t burn very intensely or very quickly. And then there’s the so-called critically dry vegetation, which essentially burns in the blink of an eye and with great intensity, allowing for all of these cascading positive feedbacks in the form of increasingly extreme and exotic fire behavior, which of course are further amplified when hurricane-like fires are encountered on these critically dry fuels Winds affect, as was the case this week.

I saw you mention something called “atmospheric thirst” but wasn’t sure what you meant. Is this related to what you’re talking about?

That is the case, and the main connection between the increasing risk of wildfires and climate change is actually the same process, the drought or dryness of the vegetation itself, which of course ultimately becomes the fuel for wildfires. By definition, a wildfire is a fire that burns vegetation. And there are really two ways to dry out vegetation pretty quickly. One of them is not supplying water on the supply side. In other words: If there is too little rain or snow, i.e. too little precipitation, the soil becomes dry and the plants don’t have any water to work with.

There is also another way, namely, even when there is a lot of precipitation, but an excess of evaporation and transpiration – where evaporation is the passive part of the process and transpiration is the part of the process that occurs when water flows through living plants and comes out The stomata in the leaves create something known as atmospheric evaporative demand. This is the technical term for thirst and essentially represents the tendency of the air to draw water from surfaces or plants or to evaporate. Surfaces can be bodies of water or the ground. The soil usually contains at least some water, i.e. water that can potentially be evaporated. Evaporation increases faster the greater the need for evaporation or the thirst.